It's not a fill plug in the usual sense. It's a drain plug that has yet another plug tapped into the center of it. As it turns out, that center plug has 1/8" pipe threads, so it seems like you can make a simple adapter by going to Home Depot/Checker/whatever and getting a 1/8" pipe thread with a hose barb fitting, and then using a simple pump (also at Checker Auto) to pump in the new ATF. I confirmed the fit of the barb tonight, and will see if it all works tomorrow, but this fitting looks just like the one posted so I can't imagine why it wouldn't work.
The downside is that hand-pumping 12 qts of ATF just doesn't sound like any fun at all. I've seen posted (in the pre-02 Explorer forum) a discussion of taking the two hoses from the tranny to the cooler and putting each in containers--one empty to catch the old fluid and the other full to suck in the new ATF. But there seemed to be a debate there whether there was really enough suction to pull in the new ATF. I have to admit that I'm dubious, since I'm guessing the tranny pump is designed to push rather than suck (no jokes about these trannys sucking!). Does anyone know about this??
BTW, the other problem with the process is that it's nearly impossible to remove the hoses from the transmission cooler (or at least that's the case on my V8 with, supposedly, a heavy duty tranny cooler.) The Haynes manual says you need to remove the radiator to get to the tranny fittings, and after staring at it for quite awhile, I think they're right. I'm adding a tranny filter, so I'm cutting the hoses, which could make it possible. But after that, I think the only good option would be to use the fittings at the tranny itself. At that point, I'm willing to pay the dealer or local tranny shop the $120.